1. Technical Field
The present invention relates to an automatic gain control device for satellite positioning receivers. It is notably applicable to Global Navigation Satellite Systems, or GNSS, and may be implemented in any navigation receiver.
2. Discussion on the Background
A satellite positioning system, or GNSS, comprises a plurality of signal transmitters disposed on as many satellites forming a constellation. Examples of satellite navigation systems of the GNSS type may be mentioned, notably: the system commonly denoted by the acronym “GPS” for “Global Positioning System”, and the system commonly denoted “Galileo”. A minimum of four positioning satellites allow a mobile receiver capable of processing the received signals coming from the latter to deliver position data from the receiver, in terms of geographical coordinates (x,y,z) at a given time instant t. The data transmitted by the positioning satellites occupy a wider bandwidth than that required by the data rate to be transmitted, with the aim of reducing the influence of interfering signals, and of reducing the power levels of the transmitted signals in such a manner that the latter are masked in the background noise. Thus, according to techniques known per se, the spectrum of the transmitted signals is a spread spectrum, a carrier wave being modulated by a data signal superimposed onto a high-frequency pseudo-random noise spread signal. A correlation of the received signals with local codes allows the useful signal to be extracted from the noise. The correlation function of a GNSS receiver requires a signal-to-noise ratio that is sufficient to allow the demodulation of the data, and to allow code and phase tracking of the signal.
A certain number of frequency bands are allocated to navigation systems of the GNSS type. Recently, new frequency bands have been allocated to systems of the GPS type referred to as “modernized” GPS systems and to the Galileo system, respectively: the frequency bands L5 and E5a, E5b. The latter frequency bands have the particular feature of already being allocated to pulsed radio-navigation systems, amongst which the system commonly denoted by the acronym DME, corresponding to “Distance Measuring Equipment”, may notably be mentioned. Furthermore, these frequency bands are also at the limit of a frequency band allocated to radar systems, amongst which are radar systems such as meteorological radar or primary radar systems typically operating in a frequency band adjacent to the E5b band.
Thus, from the point of view of a satellite navigation receiver of the GNSS type, and notably of the modernized GPS or Galileo type, the signals coming from radio-navigation systems such as DME, or from radars, are respectively considered as interference in the frequency bands of interest, or as out-of-band interference. The aforementioned radio-navigation and radar systems are usually fixed on the ground, and an aircraft may be subjected to various types of pulsed interference depending on the areas over which it flies. Furthermore, this pulsed interference is very powerful in transmission; thus, when an aircraft overflies for example the European and American continents, within which there is a high concentration of sources of interferences, the occupancy rate of the pulses can be high, even greater than 100% where there is overlapping of the pulsed interference sources.
As regards the protection against interference associated with the presence of undesirable signals, the aforementioned interference sources present difficulties in the time processing by the GNSS receiver. Notably, a GNSS receiver comprises a system for Adaptive Gain Control, commonly denoted by the acronym AGC, acting on the radiofrequency signal received via an antenna and after pre-amplification, allowing the coding of the input radiofrequency signal to be optimized downstream within the dynamic range of an Analogue-Digital Converter, commonly denoted by the acronym ADC. Certain pulsed interference scenarios destabilize the AGC and render difficult or even impossible any digital processing downstream of the ADC.
The GNSS receivers must consequently be robust to pulsed interference sources in the GNSS useful band with high repetition rates, which correspond to pulsed interference scenarios for example of the DME type, and also be robust to high-power pulsed interference outside of the useful band, notably produced by radar systems.
One known solution that aims to enable GNSS receivers to overcome interference effects caused by pulses is the technique denoted by the term “blanking”, which consists in identifying the interfering signal and in eliminating the received signal affected by the latter from the later processing operations. This solution is unworkable when the interference density increases to the point of covering the useful signal virtually continuously. In this case, the blanking leads to any useful signal being eliminated at the same time as the interfering signal. This type of scenario is likely to happen in a large part of the European air space, notably at an altitude of around 40,000 feet where the number of DME beacons seen by an aircraft can be around 60 at the times of maximum density of traffic. In order to improve the efficiency of the blanking, the band can be divided up into several sub-bands and the blanking applied over each of the sub-bands, which, for a given level of interference, allows a larger part of the useful signal to remain and hence improves the signal-to-noise ratio.
In both cases, a noise reference needs to be available which allows the bias in estimation of the thermal noise, which appears in the dense interference scenarios, to be overcome. One solution consists in calibrating a noise reference; however, this solution is not stable either in time or in temperature, or with respect to the dynamic processes to which the receiver is subjected.
Thus, according to an improved technique described in the Patent application published under the reference FR 2,916,589, an estimation of the thermal noise is provided without recourse to the calibration. According to this technique, the AGC is closed-loop controlled by the probability density function of the power or of the amplitude of the radiofrequency signal, and is based on the principle that the left-hand part of the probability density curve as a function of the power or of the amplitude is not or is hardly affected by interference pulses within the useful band. This technique turns out to be very effective for pulses within the useful band, even for high repetition rates. However, this technique proves to be less effective with regard to scenarios of high out-of-band pulse occupancy rates, saturating the pre-amplifier that operates over the whole L band. When the latter is saturated by a pulse within the L band but outside of the useful band, its gain collapses over the whole L range. The AGC estimator is thus biased and the useful signal is therefore no longer amplified, and it becomes difficult or even impossible to extract it from the noise by correlation.